The present invention relates generally to dielectric filters, and, more particularly, to a dielectric filter assembly having a mounting bracket which mounts upon a substrate to electrically connect the dielectric filter to a circuit disposed upon the substrate.
Filter circuitry which filters an input signal applied thereto of undesired frequency components is well known. For instance, designs of filter circuitry to form band pass, band reject, low pass and high pass filters are all well known. Such filter circuitry permits passage of, or rejection of, certain frequency component portions of a signal applied to the filter circuitry. A large variety of electrical circuit constructions include, as portions thereof, filter circuits formed of such filter circuits.
Filter circuitry is typically classified to be of either of two types of construction: active or passive. Active filter circuits are advantageously disposed upon integrated circuits and, hence, are of minimal sizes. However, active filter circuits are generally linear over only a limited dynamic range. Such active filter circuits also generally exhibit desired filter characteristics over only the limited dynamic range in which the active filter circuit is linear.
passive filter circuits, conversely, exhibit proper filter characteristics over a greater dynamic range. Passive filter circuits are comprised of passive filter components, namely, combinations of resistors, capacitors, and inductors. The resistive, capacitive, and inductive component values of such filter components, and their respective electrical connections therebetween, define a resonant frequency. By appropriate connection of the passive filter components, any of the above-listed filter circuits may be formed.
A passive filter circuit positioned in a series connection, for instance, with an electrical circuit forms a band pass filter which passes signal portions of a signal applied thereto which are within a range of frequencies defined by the resonant frequency of the filter circuit. The resonant frequency of the passive filter circuit is defined by the component values of the passive filter components. Appropriate selection of the values of the passive filter components causes the passive filter circuit to for a pass band of a desired bandwidth and a desired center frequency. Combinations of such series-connected, passive filter circuits may be formed to pass signal portions of any selected range of frequencies.
A passive filter circuit positioned in a shunt connection with an electrical circuit forms a band reject filter wherein signal portions of a signal applied thereto which are within a range of frequencies defined by the resonant frequency of a filter circuit are shunted, and are not passed by the filter circuit. The resonant frequency of the passive filter circuit is, again, defined by the component values of the passive filter components. Appropriate selection of the values of the passive filter components causes the passive filter circuits to form a reject band of a desired bandwidth and a desired center frequency.
Combinations of both the passive filter circuits connected in the series connection, and the passive filter circuits connected in the shunt connection may, of course, be formed to perform circuit functions as desired.
As mentioned hereinabove, a large variety of electrical circuits comprise passive filter circuits forming portions thereof. One such electrical circuit is a radio-frequency receiver circuit. Passive filter circuitry is utilized, for example, to tune the receiver, and to filter intermodulation spurs generated during down conversion and demodulation of a signal received by the receiver circuit. Additionally, passive filter circuitry is utilized to form portions of a receiver circuit to prevent passage of other spurious signals generated during down conversion of a signal received by a receiver circuit. Filter circuitry is additionally, of course, utilized to form portions of a receiver circuit to perform other filter functions.
Ceramic and other dielectric materials are often utilized to form a passive filter circuit. Passive filter circuits constructed of such materials are commonly referred to as "ceramic block filters" because of the geometric configuration of such filters. conventionally, a ceramic block filter is formed in the shape of a block, and one or more holes are drilled or otherwise formed to extend through the block. Such holes form resonating cavities which resonate at frequencies determined by the length of the cavity. Portions of the sidewalls defining the cavity are typically coated with an electrically-conductive material, such as a silver-containing compound. Portions of surfaces, or entire surfaces, of the ceramic block are also typically covered with the electrically-conductive material.
The resonating frequency of the resonating cavity formed of such holes is additionally dependant upon the surface area of the sidewalls defining such cavities.
Ceramic block filters and/or apparatus for connecting such filters to an electrical circuit are disclosed in U.S. Pat. Nos. 4,431,977; 4,673,902; 4,703,921; 4,716,391; and 4,742,562.
Transceivers, such as portable, cellular phones, oftentimes utilize such ceramic block filters. Electrical circuits of such portable transceivers include both receiver portions and transmitter portions, each of which includes one or more ceramic block filters to form filter circuits. Such ceramic block filters, for example, filter signal portions of signals received by the receiver circuitry, and filter signal portions of signals generated by the transmitter circuitry. A ceramic block filter may, for instance, form an interstage filter positioned between stages of the transmitter and/or receiver circuit of the portable transceiver, or form a duplex filter positioned between the receiver circuitry and an antenna, and between the antenna and the transmitter circuitry of the transceiver.
Typically, a ceramic block filter is mounted upon a circuit board, such as a printed circuit board, and is suitably connected to an electrical circuit disposed or mounted, thereupon. Conventionally, circuit elements such as a ceramic block filter are positioned at desired locations upon a printed circuit board. The printed circuit board, containing the desired circuit elements is then placed in a bath of molten solder. Portions of the printed circuit board are coated thereby with the solder material to affix in position the circuit elements thereupon. Such a method is referred to as a waveline solder process.
When utilizing such a solder technique, a monolithic element, such as the ceramic block filter, is susceptible to movement, i.e., the filter may "float" as the solder material liquefies during the solder operation. Therefore, conventionally, the ceramic block filter is first disposed upon, or placed within, a mounting bracket, and the mounting bracket is mounted upon the circuit board to be affixed thereto by the reflow solder operation.
U.S. Pat. No. 4,716,391, mentioned briefly hereinabove, discloses one such ceramic filter and mounting bracket therefor. Glass feed-thru pins electrically connect input and output electrodes formed upon a face surface of the ceramic block filter. The feed-thru pins are inserted into openings formed in the mounting bracket to abut against the ceramic block filter. Once positioned, the feed-thru pins are soldered to the electrode of the ceramic block filter and to the printed circuit board by the waveline solder process.
When using such a filter and bracket combination, care must be exercised to ensure that the pins do not become loose and fall out of the openings defined by the resonating cavities prior to soldering thereto. Also, care must be exercised to ensure that excessive amounts of solder are not applied to the pins as short circuiting of the ceramic block filter could result.
Most significantly, however, a waveline solder process leaves a residue of solder flux on the circuit board which must be removed from the circuit board. Most easily, the solder flux may be removed by application of a FREON (.TM.)-based solvent to the printed circuit board. However, because of environmental concerns regarding the use of FREON-based compounds, application of such a solvent to remove solder flux from a circuit board is to be discontinued.
Methods and apparatus for electrically-connecting ceramic block filters to a printed circuit board have been developed which utilize reflow solder techniques. A reflow solder technique is one in which small amounts of solder material are placed upon surface areas of the printed circuit board and circuit elements which are to be soldered theretogether. The circuit board is then elevated in temperature (such as placement thereof in an oven) to liquefy the solder material to solder thereby the circuit elements to the circuit board.
What is needed, therefore, is a dielectric block filter assembly having a mounting block which is permitting of electrical connection to a printed circuit board to be affixed in position thereat by use of a reflow solder technique.